Stimulated Raman scattering (“SRS”) has a rich and evolving history since the development of the laser. In 1962, SRS effect at infrared frequencies was discovered. This discovery was soon described as a two-photon process with a full quantum mechanical calculation. To account for anti-Stokes generation and higher-order Raman effects, however, coupled-wave formalism was adopted to describe the stimulated Raman effect. Self-focusing was later included to account for the much larger gain observed in SRS. These understandings facilitated the study and design of Raman amplifiers and lasers. For example, low-threshold microcavity Raman lasers have been demonstrated in silica micro spheres and micro disks using excited whispering gallery modes (“WGMs”). Such devices can play an important role in the developing technology of photonic integrated circuits.
Because silicon is being considered as a promising platform for photonic integrated circuits, silicon based photonic devices have been increasingly researched. Microscopic passive silicon photonic devices such as bends, splitters, and filters have been developed. Active functionalities in highly integrated silicon devices have been studied, such as optical bistability due to the nonlinear thermal-optical effect and fast all-optical switching with two-photon absorption.
Silicon based Raman amplifiers and lasers also have been studied. The bulk Raman gain coefficient gR in silicon is 104 times higher than in silica. Light generation and amplification in planar silicon waveguides with Raman effects have been studied recently. Raman lasing using a silicon waveguide as the gain medium has been demonstrated, where the ring laser cavity is formed by an 8-m-long optical fiber. A Raman laser using an S-shaped 4.8-cm-long silicon waveguide cavity with multi-layer coatings has also been reported, which could be integrated onto CMOS-compatible silicon chips.
Despite these advances, microscopic low-threshold Raman amplification and lasing devices on a monolithic silicon chip has yet to be developed. Such devices would support the development towards efficient, all-optical photonic integrated circuits.